“Except at those public crossings indicated in special instruction, no part of a movement may be allowed to stand on any part of a public crossing at grade, for a longer period than 5 minutes, when vehicular or pedestrian traffic requires passage. Switching operations at such crossing must not obstruct vehicular or pedestrian traffic for a longer period than 5 minutes at a time. When emergency vehicles require passage, employees must cooperate to quickly clear the involved crossings.”